1. Field of the Invention
The present invention relates to a thrust member of a motor, which is attached to an outer surface of a motor body, and which presses against a rear end face of a rotary shaft, and further to a motor provided with such a thrust member.
2. Description of the Related Art
A worm gear may be attached onto a rotary shaft of a motor, such as a stepping motor, so as to rotate a worm wheel at an appropriate reduction ratio. However, when a worm gear is attached onto a rotary shaft and meshes with an outside gear, such as a worm wheel, for speed reduction, the rotary shaft receives a large axial force. The direction of the axial force is reversed when the motor reverses its rotation direction. Since a stepping motor is usually given with an axial play on its rotor, it is necessary to prevent axial movement of the rotor fixedly attached onto the rotor shaft. Preventive measures to prohibit the rotor from axially moving are disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. H10-290548, Unexamined Japanese Patent Application KOKAI Publication No. 2001-309610, and Unexamined Japanese Utility Model Application KOKAI Publication No. H04-134163.
Unexamined Japanese Patent Application KOKAI Publication No. H10-290548 discloses a small motor which has a coil spring disposed thereinside adjacent to its rear end wall, and which presses its rotary shaft frontward. In this connection, the coil spring may be replaced by a plate spring. A coil or plate spring attached inside a motor generates only a small force to press a rotary shaft frontward, which cannot successfully counter an axial force of a rotary shaft moving rearward by means of a worm gear meshing with an outside gear. In this case, the rotor hits against a bearing, which supports the rearward portion of the rotary shaft, or hits against other members, thereby generating axial vibration. Also, since the rotor makes surface contact with the bearing or other members when hitting against them, torque is decreased due to friction generated by the surface contact. Further, in case of a stepping motor, the axial vibration is generated every time when a stepping motion is made. The spring force of the coil or plate spring can be increased in order to duly counter the rearward moving force of the rotary shaft. This, however, causes torque to decrease due to the very increase of the spring force, when the rotary shaft is not moved axially, for example, when the worm gear is not attached.
Unexamined Japanese Patent Application KOKAI Publication No. 2001-309610 discloses a motor attaching structure in which the front end portion of a rotary shaft having a worm gear attached there-onto is blocked so as to inhibit the worm gear from coming off from the rotary shaft, and Unexamined Japanese Utility Model Application KOKAI Publication No. H04-134163 discloses a gear case which is attached to the front end portion of a rotary shaft and inhibits axial movement of the rotary shaft. These approaches, however, help only when the rotary shaft rotates in one direction so as to be moved frontward by means of a worm gear, and are of no help when the rotary shaft reverses its directional direction so as to be moved rearward by the worm gear, and additional approaches are needed thus complicating the structure of the motor.